1. Field of the Invention
The present invention relates to a printing apparatus and method for performing printing using a printhead having a plurality of printing elements arrayed in rows. More particularly, the invention relates to a printing apparatus and method ideal in a case where use is made of a so-called full-line-type ink-jet printhead.
2. Description of the Related Art
A printing apparatus used as a printer or copier, etc., or as the output device of a multifunction electronic device or work station inclusive of a computer or word processor, etc., is adapted to print an image (inclusive of characters and symbols) on a printing medium such as printing paper based upon print information. Such printing apparatuses are classified into those that rely upon ink-jet, wire-dot, thermal and laser printing schemes.
In a so-called serial-type printing apparatus that prints while scanning in a main-scan direction that intersects the conveyance direction (sub-scan direction) of a printing medium, an image is printed using a printhead serving as printing means moved along the printing medium. That is, printing is performed over the entire area of the printing medium by repeating an operation of conveying the printing medium a prescribed amount at a time whenever one main scan of printing is completed by the printhead.
In a so-called full-line-type printing apparatus in which the printhead has a printing width that corresponds to the width of the printing medium so that the only motion is in the conveyance direction of the printing medium, the printing medium is placed at a prescribed position and printing is performed one line at a time while the printing medium is conveyed. Printing over the entire area of the printing medium is thus performed.
Among these types of printing apparatuses, the ink-jet printing apparatus, which uses an ink-jet printhead as printing means and performs printing by discharging ink toward the printing medium from discharge orifices of the printhead, is advantageous in that it lends itself to size reduction of the printhead, is capable of forming a high-definition image at high speed and features low running cost since it is capable of printing on so-called plain paper without requiring special processing. Additional advantages are low noise owing to non-impact printing and the ability to form a color image using inks of multiple colors. Furthermore, these types of printing apparatus include a full-line printing apparatus that uses a so-called full-line-type printhead in which a number of ink-jet printing elements are arrayed to intersect (generally at a right angle) the conveyance direction of the printing medium. The full-line printing apparatus is capable of forming images at much higher speeds and has become noteworthy owing to the possibility of use as an on-demand printing apparatus for which there is increasing need at present. (By way of example, see the specification of Japanese Patent Application Laid-Open No. 2002-292859). The ink-jet printing elements are situated along the full width of the printing area of the printing medium and are capable of discharging ink from discharge orifices.
In a full-line printing apparatus for such on-demand printing, monochrome printing such as the printing of text requires printing on a printing medium of size A3 at a rate of 30 pages per minute or greater with a resolution of 600×600 dpi (dots per inch) or greater.
Further, printing of a full-color image such as a photograph requires printing on a printing medium of size A3 at a rate of 30 pages per minute or greater with a resolution of 1200×1200 or greater.
However, in the case of a printhead used in the above-described full-line printing apparatus, it is difficult to machine, without defects, all of the ink-jet printing elements situated across the full width of the printing area of the printing medium. It is particularly difficult to machine, without defects, all of the discharge orifices that constitute part of the ink-jet printing elements. For example, in order to print on size A3 paper with a resolution of 1200 dpi in a full-line printing apparatus, it is necessary to form about 14,000 discharge orifices in the full-line printhead (across a printing width of about 280 mm). Further, in terms of the manufacturing process of a full-line printhead, it is difficult to machine all of the ink-jet printing elements corresponding to this multiplicity of discharge orifices without a single defect. In addition, even if such a printhead could be manufactured, the success rate would be low and manufacturing cost high.
For these reasons, a so-called connecting head has been proposed as a full-line printhead used in a full-line ink-jet printing apparatus. The connecting head includes a plurality of printhead chips each having a printing element array in each of which a plurality of printing elements have been arrayed. The plurality of printhead chips are arrayed in the direction of the printing element arrays. In the connecting head, a plurality of comparatively inexpensive short chips of the kind used in the printhead of the serial-type printing apparatus are joined together in the direction in which the discharge orifices are arrayed. An increase in length of the printhead is achieved by arraying these chips with high precision.
However, as a result of research, the inventors have clarified that because of the structure of the connecting head, a decline in image quality, namely the appearance of a so-called “connecting line”, tends to occur at printed portions printed by the printing elements situated at the joints between the plurality of chips. The joints are at locations where there is overlap between the end portions of the printing element arrays of mutually adjacent printhead chips. More specifically, conveyance of the printing medium in the printing apparatus tends to vary (this is so-called “conveyance meandering”), and a tilt develops in the relative positional relationship between the full-line printhead and printing medium. Because of these effects, the discharge orifice pitch formed by the discharge orifices of the mutually adjacent printing elements at the joint is not the same as that of the other discharge orifices, and a line (the above-mentioned connecting line) corresponding to the joint between the chips appears. Owing to the structure of the connecting head, there are many cases where the distance between the rows of discharge orifices in the direction of these rows used in printing at the joints is greater than that of the rows of discharge orifices used in printing at the portions where joints are absent. As a consequence, it is believed that a contributing factor to the decline in image quality is that printing is readily susceptible to the effects of conveyance meandering or inclination at the joints.
The above-mentioned Japanese Patent Application Laid-Open No. 2002-292859 has also been disclosed as means for dealing with connecting lines produced by the connecting head, and several improvements have been proposed thus far. For example, there is a method of reducing a difference in the pitch of the discharge orifices using an arraying method or arraying apparatus for arraying the chips at the joints in a highly accurate manner. Another method for dealing with the problem is to array a prescribed number each of discharge orifices at the ends of the chips so as to overlap in the conveyance direction of the printing medium at the joints between chips without arraying the discharge orifices at the ends of the chips such that they will be adjacent in the array direction of the orifices. In this case, connecting lines are rendered inconspicuous by discharging ink from both of the mutually overlapping discharge orifices at the time of printing.
Nevertheless, these measures are not fully satisfactory for dealing with connecting lines that occur when performing photograph-like printing.
As a result of research, the inventors have discovered that there are cases where connecting lines are reduced by adopting multiple printing element arrays on the printhead chip of a full-line printhead. In most cases, however, good results are not obtained merely by adopting multiple printing element arrays. Accordingly, by conducting further extensive research with regard to how to adopt multiple printing element arrays on printhead chips in order to reduce connecting lines, the inventors have completed the present invention.